1. Field of the Invention
The present invention relates to the seaming of multiaxial fabrics on a papermaking machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
The present invention relates primarily to the fabrics used in the press section, generally known as press fabrics, but it may also find application in the fabrics used in the forming and dryer sections, as well as in those used as bases for polymer-coated paper industry process belts, such as, for example, long nip press belts.
Press fabrics play a critical role during the paper manufacturing process. One of their functions, as implied above, is to support and to carry the paper product being manufactured through the press nips.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.
Perhaps most importantly, the press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to fulfill this function, there literally must be space, commonly referred to as void volume, within the press fabric for the water to go, and the fabric must have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
Contemporary press fabrics are used in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a woven base fabric into which has been needled a batting of fine, non-woven fibrous material. The base fabrics may be woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered, multi-layered or laminated. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam. Alternatively, they may be produced by a process commonly known as modified endless weaving, wherein the widthwise edges of the base fabric are provided with seaming loops using the machine-direction (MD) yarns thereof. In this process, the MD yarns weave continuously back and forth between the widthwise edges of the fabric, at each edge turning back and forming a seaming loop. A base fabric produced in this fashion is placed into endless form during installation on a paper machine, and for this reason is referred to as an on-machine-seamable fabric. To place such a fabric into endless form, the two widthwise edges are seamed together. To facilitate seaming, many current fabrics have seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are often formed by the machine-direction (MD) yarns of the fabric. The seam is typically formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together.
Further, the woven base fabrics may be laminated by placing one base fabric within the endless loop formed by another, and by needling a staple fiber batting through both base fabrics to join them to one another. One or both woven base fabrics may be of the on-machine-seamable type.
In any event, the woven base fabrics are in the form of endless loops, or are seamable into such forms, having a specific length, measured longitudinally therearound, and a specific width, measured transversely thereacross. Because paper machine configurations vary widely, paper machine clothing manufacturers are required to produce press fabrics, and other paper machine clothing, to the dimensions required to fit particular positions in the paper machines of their customers. Needless to say, this requirement makes it difficult to streamline the manufacturing process, as each press fabric must typically be made to order.
Fabrics in modern papermaking machines may have a width of from 5 to over 33 feet, a length of from 40 to over 400 feet and weigh from approximately 100 to over 3,000 pounds. These fabrics wear out and require replacement. Replacement of fabrics often involves taking the machine out of service, removing the worn fabric, setting up to install a fabric and installing the new fabric. While many fabrics are endless, about half of those used in press sections of the paper machines today are on-machine-seamable. Some Paper Industry Process Belts (PIPBs) are contemplated to have an on machine seam capability, such as some transfer belts, known as Transbelt®. Installation of the fabric includes pulling the fabric body onto a machine and joining the fabric ends to form an endless belt.
In response to this need to produce press fabrics in a variety of lengths and widths more quickly and efficiently, press fabrics have been produced in recent years using a spiral winding technique disclosed in commonly assigned U.S. Pat. No. 5,360,656 to Rexfelt et al., the teachings of which are incorporated herein by reference.
U.S. Pat. No. 5,360,656 shows a press fabric comprising a base fabric having one or more layers of staple fiber material needled thereinto. The base fabric comprises at least one layer composed of a spirally wound strip of woven fabric having a width which is smaller than the width of the base fabric. The base fabric is endless in the longitudinal, or machine, direction. Lengthwise threads of the spirally wound strip make an angle with the longitudinal direction of the press fabric. The strip of woven fabric may be flat-woven on a loom which is narrower than those typically used in the production of paper machine clothing.
The base fabric comprises a plurality of spirally wound and joined turns of the relatively narrow woven fabric strip. The fabric strip is woven from lengthwise (warp) and crosswise (filling) yarns. Adjacent turns of the spirally wound fabric strip may be abutted against one another, and the spirally continuous seam so produced may be closed by sewing, stitching, melting, welding (e.g. ultrasonic) or gluing. Alternatively, adjacent longitudinal edge portions of adjoining spiral turns may be arranged overlappingly, so long as the edges have a reduced thickness, so as not to give rise to an increased thickness in the area of the overlap. Alternatively still, the spacing between lengthwise yarns may be increased at the edges of the strip, so that, when adjoining spiral turns are arranged overlappingly, there may be an unchanged spacing between lengthwise threads in the area of the overlap.
In any case, a base fabric, taking the form of an endless loop and having an inner surface, a longitudinal (machine) direction and a transverse (cross-machine) direction, is the result. The lateral edges of the base fabric are then trimmed to render them parallel to its longitudinal (machine) direction. The angle between the machine direction of the base fabric and the spirally continuous seam may be relatively small, that is, typically less than 10°. By the same token, the lengthwise (warp) yarns of the fabric strip make the same relatively small angle with the longitudinal (machine) direction of the base fabric. Similarly, the crosswise (filling) yarns of the fabric strip, being substantially perpendicular to the lengthwise (warp) yarns, make the same relatively small angle with the transverse (cross-machine) direction of the base fabric. Note, the crosswise and lengthwise yarns in the fabric strip may slip such that they are not always perpendicular to one another. In short, neither the lengthwise (warp) nor the crosswise (filling) yarns of the fabric strip align with the longitudinal (machine) or transverse (cross-machine) directions of the base fabric.
A press fabric having such a base fabric may be referred to as a multiaxial press fabric. Whereas the standard press fabrics of the prior art have three axes: one in the machine direction (MD), one in the cross-machine direction (CD), and one in the z-direction, which is through the thickness of the fabric, a multiaxial press fabric has not only these three axes, but also has at least two more axes defined by the directions of the yarn systems in its spirally wound layer or layers. Moreover, there are multiple flow paths in the z-direction of a multiaxial press fabric. As a consequence, a multiaxial press fabric has at least five axes. Because of its multiaxial structure, a multiaxial press fabric having more than one layer exhibits superior resistance to nesting and/or to collapse in response to compression in a press nip during the papermaking process as compared to one having base fabric layers whose yarn systems are parallel to one another.
Until recently, multiaxial press fabrics of the foregoing type had been produced only in endless form. As such, their use had been limited to press sections having cantilevered press rolls and other components, which permit an endless press fabric to be installed from the side of the press section. However, their relative ease of manufacture and superior resistance to compaction contributed to an increased interest and a growing need for a multiaxial press fabric which could be seamed into endless form during installation on a press section, thereby making such press fabric available for use on paper machines lacking cantilevered components. On-machine-seamable multiaxial press fabrics, developed to meet this need, are shown in commonly assigned U.S. Pat. Nos. 5,916,421; 5,939,176; and 6,117,274 to Yook, the teachings of which are incorporated herein by reference.
U.S. Pat. No. 5,916,421 shows an on-machine-seamable multiaxial press fabric for the press section of a paper machine made from a base fabric layer assembled by spirally winding a fabric strip in a plurality of contiguous turns, each of which abuts against and is attached to those adjacent thereto. The resulting endless base fabric layer is flattened to produce first and second plies joined to one another at folds at their widthwise edges. Crosswise yarns are removed from each turn of the fabric strip at folds at the widthwise edges to produce unbound sections of lengthwise yarns. A seaming element, having seaming loops along one of its widthwise edges, is disposed between the first and second fabric plies at each of the folds at the two widthwise edges of the flattened base fabric layer. The seaming loops extend outwardly between the unbound sections of the lengthwise yarns from between the first and second fabric plies. The first and second fabric plies are laminated to one another by needling staple fiber batting material therethrough. The press fabric is joined into endless form during installation on a paper machine by directing a pintle through the passage formed by the interdigitation of the seaming loops at the two widthwise edges.
U.S. Pat. No. 5,939,176 also shows an on-machine-seamable multiaxial press fabric. Again, the press fabric is made from a base fabric layer assembled by spirally winding a fabric strip in a plurality of contiguous turns, each of which abuts against and is attached to those adjacent thereto. The resulting endless fabric layer is flattened to produce a first and second fabric plies joined to one another at folds at their widthwise edges. Crosswise yarns are removed from each turn of the fabric strip at the folds at the widthwise edges to produce seaming loops. The first and second plies are laminated to one another by needling staple fiber batting material therethrough. The press fabric is joined into endless form during installation on a paper machine by directing a pintle through the passage formed by the interdigitation of the seaming loops at the two widthwise edges.
Finally, in U.S. Pat. No. 6,117,274, another on-machine-seamable multiaxial press fabric is shown. Again, the press fabric is made from a base fabric layer assembled by spirally winding a fabric strip in a plurality of contiguous turns, each of which abuts against and is attached to those adjacent thereto. The resulting endless fabric layer is flattened to produce a first and second fabric plies joined to one another at folds at their widthwise edges. Crosswise yarns are removed from each turn of the fabric strip at the folds at the widthwise edges to produce unbound sections of lengthwise yarns. Subsequently, an on-machine-seamable base fabric, having seaming loops along its widthwise edges, is disposed between the first and second fabric plies of the flattened base fabric layer. The seaming loops extend outwardly between the unbound sections of the lengthwise yarns from between the first and second fabric plies. The first fabric ply, the on-machine-seamable base fabric and the second fabric ply are laminated to one another by needling staple fiber batting material therethrough. The press fabric is joined into endless form during installation on a paper machine by directing a pintle through the passage formed by the interdigitation of the seaming loops at the two widthwise edges.
A seam is generally a critical part of a seamed fabric, since uniform paper quality, low marking and excellent runnability of the fabric require a seam which is as similar as possible to the rest of the fabric in respect of properties such as thickness, structure, strength, permeability etc. It is important that the seam region of any workable fabric behave under load and have the same permeability to water and to air as the rest of the fabric, thereby preventing periodic marking of the paper product being manufactured by the seam region. Despite the considerable technical obstacles presented by these seaming requirements, it is highly desirable to develop seamable fabrics, because of the comparative ease and safety with which they can be installed.
As discussed above in reference to U.S. Pat. No. 5,939,176, a CD area of the multiaxial fabric is raveled out and the fabric is then folded over in this raveled area to produce seaming loops. A drawback to this approach of creating a seam in the multiaxial fabric structure is the CD yarn tails that result in the seam area. These tails are a function of the CD yarn angle which is linked to the panel width, fabric length and panel skew. These yarn tails are not anchored into the base weave and are free to move or “migrate” into the seam area. This problem is known as yarn migration. When this migration occurs, the CD ends move into the seam area and impede seaming (sometimes significantly). In addition, these unbound yarns do not provide suitable uniform support for the fiber batting material in the seam area.
Attempts have been made to use certain adhesives to bind these yarns and prevent migration, but with limited success. Therefore, a need exists for an improved seam to prevent yarn migration in multiaxial fabrics.